When a brake device for generating a braking force by operation of a motor without using hydraulic means on a plurality of wheels of a vehicle, it is necessary to control the braking force of each wheel separately. Accordingly, if the braking force control is not sufficiently accurate, there is a possibility that a braking force difference may be caused to make the behavior of each wheel unstable. For example, when a braking force difference is caused between the right and the left wheel of a vehicle, a yaw angular velocity is generated upon brake in the vehicle body and a passenger feels unpleasant feeling. Moreover, in the worst case, the vehicle body may spin.
It should be noted that in a case of a train, the right and the left wheel are connected to each other in general, and the braking force difference between the right and the left wheel will not cause a significant problem.
In general, in case of a disc brake, the wheel braking force is determined by a force (piston thrust) pressing a brake pad against a disc rotor. Consequently, in order to reduce the braking force difference between the right and the left wheel of a vehicle, it is necessary to accurately control the piston thrust. In general, the piston thrust may be controlled currently by one of the following three methods.
The first method uses a thrust sensor for measuring piston thrust mounted on an electrically operated brake and performs feedback control of the piston thrust. By using a thrust sensor of high resolution, it is possible theoretically to accurately control the piston thrust. However, vibration attributed to convex/concave portions of a road is applied to the electrically operated brake, the reliability of the thrust sensor is lowered. Moreover, when using a thrust sensor of high resolution, it is necessary to increase the resolution of an AD converter, which in turn increases the cost. Furthermore, since the temperature of the electrically operated brake is determined by the ambient temperature and the temperature of the brake pad and is not constant. Accordingly, in case of a distortion gauge type thrust sensor, the sensor temperature drift also causes a problem and the control mechanism is complicated.
In the second method, the relationship between the current flowing in the motor and the piston thrust is obtained in advance, and by referencing the graph of the motor current/piston thrust, a current corresponding to the required piston thrust is fed to the motor. Since there is a constant relationship between the motor current and the motor torque, this method is considered to be simple. Actually, however, since the transmission efficiency in the power transmission system is not 100%, during increase and decrease of the piston thrust, a hysteresis is caused, i.e., the relationship between the motor current and the piston thrust is not uniquely defined. The transmission efficiency of the power transmission system is changed by the temperature and the lubrication state of the sliding part and is not constant. Accordingly, the control of the piston thrust based only on the motor current is difficult.
In the third method, the relationship between the piston position decided by the motor position and the piston thrust is obtained in advance and by referencing the graph, the motor is controlled to the motor position corresponding to the required piston thrust. In this method, it is necessary to obtain the motor position corresponding to the contact position between the brake pad and the disc rotor and accurately estimate the rigidity of the entire system.
JP-A-11-513337 discloses a technique for the object of the aforementioned third method. The technique detects a contact position between a disc rotor and a brake pad. If the contact position of the brake pad with each wheel can be accurately detected, it is possible to reduce the braking force difference between the wheels when the third method is used.
In a first method disclosed in JP-A-11-513337, if there is a gap between the brake pad and the disc rotor, the motor is accelerated by almost constant torque in a direction to reduce the gap and the inversion of the sign of the motor acceleration caused when the brake pad is brought into contact is detected, thereby judging the contact position. Moreover, in a second method disclosed in JP-A-11-513337, a contact pin is provided in the brake pad, and when the contact pin is brought into abutment with the disc rotor, the contact pin closes a current circuit so as to generate a contact signal.